A rotary coupling for conducting a medium under pressure from a stationary to a rotating machine part can comprise a shaft which is provided with an axial service line or passage (e.g. feeding a cutting tool) and a radial connecting duct which opens into a annular passage or slot providing a feed passage for the medium. The annular passage can be sealed against the housing by appropriate sealing means.
The rotary coupling hitherto provided is a rigid or stiff system, i.e. a system in which the operational gap between the stationary and the rotating part depends in regard to its position and size exclusively on the manufacturing tolerances of the parts. No adjustment of manufactured condition and operating parameter dependent shape and position variations of this operating gap between the housing and the shaft occurs.
To maintain the hydraulic losses in the known system at an acceptable level, the gaps between the moving parts should be of the order of 5 to 10 .mu.m. These gap widths are less than the standard mounting play so that the tolerances, the thermal effects or the like can result in mechanical contact or pressing between the stationary and rotating parts and thus excessive wear at high rotation speeds.
This danger is particularly evident when a rapid acceleration to the rated speed is required. In this case as a result of unequal thermal expansion of the bushing and the shaft, a "swelling" or "growing together" can rapidly occur at the operating gap and thus the rotary coupling can be blocked.
This eventuality has considerable economic consequences to the entire system. There is a considerable danger of an accident in which hoses, connecting tubes and the like are torn.
Because of safety considerations in many case one is thus forced to increase the gap size so that manufacturing tolerances and short time thermal effects cannot lead to contact of the rotating parts with the stationary parts.
The high leakage resulting from the large gap size and the resulting high hydraulic losses is not economical and leads to considerably increased costs.
In the Swiss Patent 483 584 a rotary coupling is described in which a compensating mechanism is provided which should compensate radially exteriorly directed forces which occur during the transmission of a fluid under pressure. A large leakage rate can thereby be prevented which would otherwise lead to an increased pump capacity requirement.
In this known rotary coupling it is disadvantageous that the transmission system acts simultaneously as the mount or bearing for the shaft. The mount or bearing unit, e.g. comprising the housing and the bushing, must thus be rigid and locally fixed relative to the shaft axis. The desired compensation mechanism thus allows only an adjustment of the shaft within the sealing gap.
This system also has the considerable disadvantage that on occurrence of manufactured or operationally required large shaft displacements there is a danger that damage to the sealing and mounting gaps, even seizing or jamming, can occur.